Research into sources of mercury emissions in the U.S. has resulted in the identification of cement producing facilities as significant emission source of mercury. Currently, cement plants are the fourth largest sources of mercury emissions in the U.S. The U.S. Environmental Protection Agency (EPA) has proposed a rule to limit mercury emissions from cement plants. The proposed rule sets forth the first limits on mercury emissions from existing cement plants and strengthens the limits for new plants. The proposed rule sets mercury emission limits for existing sources at 26 pounds of mercury per million tons of feed (˜13 kg/million tons) or 43 pounds of mercury per million tons of clinker produced (˜21.5 kg/million tons). For new cement plants, the mercury emission limit is 14 pounds of mercury per million tons of clinker produced (˜7.0 kg/million tons). The proposed rule is set to take effect in 2013. The EPA estimates that when the rule is fully implemented, annual emissions of mercury from cement plants will be reduced by at least 81%.
It is known that activated carbon can be injected into a gas stream containing mercury vapor. When mercury vapor contacts activated carbon particles, the mercury is captured and held by the activated carbon particles. The particles are then collected by a particulate collection device, such as an electrostatic precipitator or a baghouse filter. The mercury captured by the activated carbon particles appears to be stably bonded to the particles. In cement plant operations, the particulates captured by the control device are normally recycled to the cement production process.
In cement plants, mercury volatilizes from the raw material. A majority of the volatilized mercury ends up in the cement dust. Typically, the exhaust gas stream from the cement production process contains particulates, and these particulates are normally collected by the particulate collection device, usually a fabric filter or an electrostatic precipitator. Collected particulates from the cement production process, which include the cement dust, are often recycled from the particulate collection device and used as part of the raw material feed, where the mercury is again volatilized as the raw material is heated. Thus, recycling the cement dust to the raw material feed continually re-introduces mercury into the cement production process and thereby into cement dust and the accompanying exhaust gas stream.
In addition, the exhaust gas itself is sometimes recycled to the cement production process. The exhaust gas typically contains a small amount of mercury; returning the exhaust gas to the raw mill introduces additional mercury to the raw material feed.
Relatively inexpensive and yet effective ways to reduce mercury emissions from cement plants are quite desirable.